1. Field of invention
The present invention relates to a continuous process, and apparatus for drying wet fibrous webs, and more particularly, it applies to the drying of wet printing papers such as newsprint. A new drying method is disclosed which, compared with the conventional method, employs a smaller number of dryers, leads to better machine runnability, and yields products with greater tensile strength, better surface properties and printing characteristics. This method is particularly suitable for drying wet cellulosic webs intended for printing products such as newsprint and bond paper.
2. Description of Prior Art
In the production of paper, a suspension of cellulosic fibres is ejected on an advancing forming fabric which retains a large portion of fibres and fine cellulosic material, and transmits a large portion of water. Additional water is removed from the wet cellulosic web by mechanical compression between two rotating press rolls. Water remaining in the pressed web is removed by evaporation in a dryer section of the paper machine.
Two elementary processes involved in paper drying are the heat transfer from the heating medium into the wet web (i.e. heat transfer) and transportation of water vapours away from the substance (i.e. mass transfer). The heat transfer is proportional to the temperature difference between the heat source and the wet web, and inversely proportional to the heat transfer resistance by the boundary layers between the web and the source of heat. An intimate contact between the heating surface and the wet web is, therefore, desirable for good heat transfer. A high temperature of the heat source is also desirable, however, the temperature of the dryer is limited by some practical considerations. For example, if the wet web is in contact with a very hot body, an insulating layer of steam is formed between the heat source and the web, and the rate of heat transfer is reduced. The high temperature of dryers could cause a reduction in paper quality. High temperature operation of steam heated dryers, also requires elevated steam pressures.
The evaporation of water from wet webs at temperatures below the boiling point of water is possible only if water vapours are carried away from the web by the drying air. The rate of this mass transfer is reduced if the web is insulated from the surrounding air by, for example, a drying fabric. On the other hand, mass transfer is enhanced by impingement of hot air on the web.
In the most common method of paper drying, the web is passed around a series of internally-heated rotating cylinders known as "dryers", which are usually arranged in an upper and a lower row. The advancing web is heated by direct contact with a portion of the cylinder surface. This drying method is well known, and is described for example in U.S. Pat. No. 2,299,460. To improve the web-dryer contact, the wet web is often sandwiched to the dryer surface by dryer fabrics. One such fabric might wrap a part of the surface of the upper row of dryers, while another dryer fabric might wrap the bottom part of the lower row of dryers.
One disadvantage associated with this method of drying is the large number of cylinders required to dry the paper. For example, 1986 survey of Canadian newsprint dryers revealed that the majority of machines operating at, or above 800 m/min had between 35 and 50 dryers, with diameters of 1.5 m or 1.8 m (N. N. Sayegh, I. I. Pikulik, and H. I. Simonsen, Pulp Paper Can, Dec. 1987).
Such a large number of dryers represents extensive capital, operating and maintenance costs, and also contributes to the great length of the machine, and to a large demand for building space. Another disadvantage of conventional paper drying methods is the transfer of unsupported, weak, wet web between two consecutive cylinders. At high machine speeds, the wet web passing unsupported through the air is unstable and, reacting to small variations in the process, has a tendency to oscillate or "flutter". An excessive sheet flutter can cause deformations and wrinkling of the sheet, reducing the product quality, or completely breaking the sheet and interrupting production. To reduce the frequency of sheet breaks, the machine speed is sometimes kept low, even though this leads to a decrease in production.
To reduce the problems associated with the movement of unsupported sheets between cylinders, on some rapidly operating paper machines, the wet paper proceeds around the initial drying cylinders adjacent to a single drying fabric. With this single felted arrangement, the paper is suported by the fabric as it advances between the dryers, which reduces the tendency of the web to flutter, and the frequecy of the breaks.
The single felted arrangement is used primarily, but not exclusively, on the initial section of cylinders where the sheet is very moist and weak, while the open draw is utilized between the remaining drying cylinders. Application of a single drying fabric in a serpentine configuration is described for example in U.S. Pat. No. 4,172,007.
A disadvantage of the single felted run is that only one half of dryers (usually those in the upper row) come into direct contact with the paper, while the other half of dryers are separated from the paper by the dryer fabric which reduces the amount of heat transferred from these dryers to wet web. Consequently, more dryers, or higher dryer temperatures are required with the serpentine arrangement of dryer felt.
Recently, a dryer section called "total Bel Run" was described in which the serpentine arrangement was extended over the whole length of the dryer section. While in the regular serpentine run the bottom row of cylinders is separated from wet web by drying fabric, in the Total Bel Run, the bottom cylinders are replaced by small diameter vacuum rolls (Beloit Canada Technical Seminar, Montreal Jan. 26, 1988). Such a dryer section is capable of operating at high speeds, but has the disadvantage of being even longer than the conventional dryer section.
Another method sometimes used for drying paper employs the "Minton Dryer" (U.S. Pat. No. 1,147,809) in which the drying cylinder is located within a large evacuated chamber. At the decreased air pressure, the boiling point of water is reduced, which could potentially increase the rate of heat transfer. The disadvantage of Minton dryers is that, in the absence of drying fabrics, the intimate contact between the dryer and the web is not established, and the rate of heat transfer is low. Another problem associated with the Minton Dryer is the necessity to disrupt the vacuum whenever a sheet break occurs. In the absence of any support for the wet web during the transfer between dryers, this method cannot be used on fast machines.
In yet another drying method, the wet web is supported by jets of heated gas which provide the heat required for the evaporation of water, and carry away water vapours. This operation is described for example in U.S. Pat. No. 3,739,491. The heat transfer rates achieved with this method are high, and the mechanical stress on the wet web is low. However, the cellulosic web dried without contact with a supporting medium shrinks unevenly and subsequently develops undesirable deviation from polarity called cockle, which lowers the product quality and might lead to wrinkles or cuts during calendering. Difficulties with threading the dryer after a web break are another disadvantage of this drying method, presently used mainly for heavy basis weight grades or for initial drying of light basis weight products.
In a different method, the web is dried entirely on a single, large-diameter, rotating, steam-heated cylinder known as "MG cylinder" or "Yankee dryer". A notable feature of web drying on a single cylinder is that the initial contact between the dryer surface and the web is established in a press nip. A metal, rubber covered press roll wrapped by a press felt or a fabric, presses the web onto the dryer surface by a force of about 30 to 80 kN/m. Upon pressing the soft wet-web fibres establish an intimate contact with the surface of the dryer which leads to a high heat transfer and drying rates. On the majority of fast modern Yankee machines, the drying rate is further enhanced by impingement of hot air on the paper adhering to the dryer surface. The jets of preheated air come from the so called "high velocity" hood, which surrounds a large portion of the Yankee dryer. Typically, about half of the drying energy is derived from the steam inside the Yankee dryer, while the other half is supplied by the hot air.
When paper is completely dried on a single, large diameter dryer, it adheres strongly to the dryer surface and cannot be safely peeled off without breaking the sheet, especially if the basis weight of the paper is low. In production of creped tissue paper, the web is dried entirely on a single Yankee dryer, and the dry product is separated from the dryer surface by a creping blade. The separated paper is densely wrinkled by the action of the blade, and usually has from 25 to 120 crepe ridges per inch. Paper creped in this manner has low tensile strength, high bulk, softness and water absorbency, and a rough surface. These properties make creped paper a good material for hygienic products, but unsuitable for application as a printing paper.
Heavier basis weight, often partially dried, cellulosic webs are sometimes also pressed to large diameter driers, called MG cylinders. These stronger and less adhesive webs might be peeled from the dryer surface, giving a product which has one side smooth and glossy, or "machine glazed" (hence MG cylinder). However, two sides of a product treated in this way are very different, namely the web side which was in contact with the dryer surface becomes smoother and glossier than the reverse side. Such a product is suitable for products such as folding boxes in which only one side is visible, while lower demands are placed on the board side inside the box. Thus Yankee drying is presently used especially for light basis weight hygienic or wrapping papers which are removed from the dryer by a creping blade, and MG cylinders are used for some paperboard products in which the difference in the two paper sides is desirable.
Recently, another method of water removal was described (U.S. Pat. No. 4,324,613) in which the cellulosic web is pressed to a cylinder heated to temperatures much higher than the boiling point of water, for example 150.degree.-250.degree. C. This process, sometimes called "Impulse Drying" is based on the generation of high pressure steam on contact of the wet web with the hot dryer surface in the press nip. The front of high pressure steam formed at the wall of the hot dryer advances rapidly through the paper thickness and expels a large proportion of liquid water contained in the cellulosic web into the adjacent felt. Since the prevalent portion of water is removed in liquid form, this process is a special case of paper pressing, rather than paper drying. Large steam pressure on the boundary of the roll and the paper causes the paper to separate from the roll immediately upon its exit from the nip. Disadvantages of paper drying include product two-sidedness and, under certain conditions, splitting of paper to two plies by high steam pressure within the sheet. Generation of steam in the press nip requires a certain nip residence time, which might limit the usefulness of this drying method for high-speed machines. No commercial high speed Impulse Drying installation exists at the present time.
The essential requirements of printing paper include good surface smoothness, identical properties of two paper sides, and resistance of the superficial fibres and fines to their removal by tacky ink during the printing process (low linting propensity). Regardless of the printing technique applied, the printing quality of paper improves with improving surface smoothness. Therefore, the smoothness of all printing papers is enhanced by calendering the dry paper in one or several nips formed by polished calender rolls. The results of calendering of paper include decreased roughness and increased gloss, which are desirable, and reduced paper thickness which is desirable only for some grades. The undesirable results include a decrease in the tensile, tear and burst strength of paper, and a reduction of the cohesion of the superficial fibres and fines with the rest of the web. Superficial material which was partially detached by the action of calender rolls, or by other means, might be removed during printing of paper by tacky ink and accumulates on the printing plates. The accumulation of this "lint" on the printing cylinder or on the printing blanket causes the appearance of undesirable print "mottle". Therefore, linting propensity is a serious defect of printing papers.
Desirable properties of printing papers include low roughness, high gloss, large tensile ant tear strength, low linting propensity and no difference in the characteristics of the two sides of paper. While smoothness and gloss of paper can be improved by calendering, this treatment has a negative effect on the strength and linting propensity of paper. Therefore, other and more expensive methods, such as the application of more expensive pulps to furnish, are sometimes used to reduce the amount of calendering required to optimize the properties of printing papers made of mechanical pulps. Clearly it is desirable to develop a process which would produce a smoother and glossier paper, especially newsprint, without negatively affecting the strength and linting properties of paper.
The equality of the surface characteristics on the two sides of paper is another important requirement of printing grades of paper. The effect on the print quality of small, but consistent deviations from the optimum values of the surface roughness, gloss, or fines content might be to some extent compensated for by modification of process parameters on the printing machine. However, a difference in the printing characteristics of the two paper sides, so called two-sidedness, results in a very noticeable and therefore undesirable difference in the print quality of two facing pages.
The importance placed by the industry on two-sidedness has been demonstrated by conversion, during the last 20 years, of the majority of newsprint formers from fourdriniers to twin-formers. The lower two-sidedness of the sheet dewatered in a more symmetrical manner on a twin-former was the main driving force for these modifications. Paper proceeding through a conventional, cylinder dryer section contacts with its alternative sides the consecutive dryers, or series of dryers in the Total Bel Run arrangement. Drying through both sides has been considered essential to prevent the development of two-sidedness. Yankee or HG dryers have not been considered suitable for printing grades of paper because they produce creped or grossly two-sided products.